1. Field of the Invention:
The present invention relates to an apparatus and method, for use with the satellite-based communications network, for improving the reliability and speed at which communication between a user terminal and the network is established. More particularly, the present invention relates to an apparatus and method for arranging data of a channel request message transmitted from a user terminal to a gateway station via a satellite in the satellite-based network to insure that critical data for establishing communication between the user terminal and the satellite-based network is received at the satellite during the appropriate receiving time frame window.
2. Description of the Related Art:
A satellite communications network, such as a geosynchronous earth orbit mobile communications network, comprises at least one geosynchronous earth orbit satellite, a ground-based advanced operations center (AOC) and spacecraft operations center (SOC) associated with the satellite, at least one ground-based gateway station (GS), and at least one user terminal (UT), which is typically a hand-held or vehicle mounted mobile telephone. The satellite enables the user terminal to communicate with other user terminals, or with other telephones in the terrestrial public switched telephone network (PSIN), via the gateway stations under the control of the gateway stations. The AOC provides system-wide resource management and control functions for its respective satellite, and the SOC controls on-orbit satellite operations for its respective satellite.
When a user terminal is operated to establish a radio resource connection, it generates and transmits a channel request message to the network on a random access channel (RACH) at a frequency assigned by the gateway station to a spot beam covering an area in which the user terminal is located. Typically, a channel request message includes data used to represent the mobile user terminal, as well as contention resolution and timing synchronization information.
To increase the likelihood that the channel request message is received at the satellite during a receiving time frame, the transmitter of the user terminal takes into account an estimated time which will elapse between the point in time when the channel request message is transmitted and the point in time when the channel request message is received at the satellite, which is known as the propagation delay. Typically, a user terminal will calculate, based on information broadcast by the network, the propagation delay for a distance measured from a location on the earth'ss surface at the center of the spot beam in which the user terminal is located to the satellite, which is orbiting at about 22,000 miles above the earth's surface. Because the distance from the surface of the earth at the equator to the satellite is less than the distance from the surface of the earth in the extreme northern and southern hemispheres to the satellite, the propagation delay for a message sent from a user terminal close to the equator is less than that for a message sent from a user terminal in, for example, northern Europe.
Also, if the user terminal is at a distance from the center of the spot beam, the propagation delay is different than at the center of the spot beam, because the distance from the user terminal to the satellite is different. However, some user terminals are equipped with global positioning system (GPS) equipment and could thus better estimate their location with respect to the center of their respective spot beam. Also, some user terminals can estimate their location with respect to the center of their respective spot beam based on the power level at which the user terminal receives the spot beam. In either instance, these type of user terminals can better estimate the propagation delay for the channel request message, and therefore increase the likelihood that the channel request message will be received at the satellite during a receiving time frame. Moreover, because the duration of a conventional channel request message is a fraction of the duration of a receiving time frame at the satellite, the entire channel request message can generally be received within the desired time frame.
To decrease call set up time, it is desirable to include in the channel request message information pertaining to the position of the user terminal making the call, the called party number, the cause for establishing the call, the service provider identification, and so on, in addition to the contention resolution and synchronization information. However, this additional information increases the overall length of the channel request message, and thus increases the likelihood that a complete channel request message will not be received by the satellite during a receiving time frame window. That is, if the instant at which the channel request message is transmitted is not accurately synchronized with the receiving time frame window, some of the information in the channel request message will not reach the satellite during the receiving time frame window, and will thus be lost. Consequently, if information necessary for call set up, such as contention resolution and timing synchronization information, is not received by the satellite, the call cannot be set up. Furthermore, even if the user terminal resends the channel request message, unless the timing difference between transmission of the channel request message and the appearance of the receiving time frames at the satellite is resolved, the network still cannot set up a call.
Accordingly, a need exists for an apparatus and method which enables a user terminal to transmit additional call set-up information in a channel request message to accelerate call set-up time between the user terminal and satellite-based communications network, while also assuring that the information necessary for call set-up is received intact by the satellite in the network.